Basically it goes like this:
Creating a single asset at a reasonable quality takes far longer than anyone thinks.
How long do you think creating a reasonably detailed 3D Model of a human being takes, given it has to be customized? A day? A week?
Most probably your estimation until your tried it yourself is WAY too low. Now, there are ways to cut down on the time needed... stock art, character generators, kitbashing parts (which is why more and more "low cost games" make a lot of use of stock art to the point game reviewer actually start seeing that as a negative trend as low cost games start to look more and more alike)...
But then, if you REALLY need a custom character, that will only reduce the time somewhat (most artists will not start from scratch, but use parts of earlier models created). Then there is the fact that many AAA games nowadays need WAY more than just a reasonably detailed character... want to show off your character models in a cutscene closeup? Well, you might have just tripled your modelling and sculpting work.
And that is just the beginning. Texturing the character, rigging and animation, voice overs, and programming all the interactions all take a lot of time. At least for an AAA title were everything is done to "perfection".
In the end, small Indie Games get away with small budgets because they work under a "good enough" mindset. They see what kind of art style could work within their limited budget, and then cut some corners to make it work. The result is "good enough", which is reasonable for a 10-15$ Indie game that most people play for its innovative gameplay or story, not the graphics glitz or voice overs.
The AAA Industry on the other hand work under the "it has to look as good as it can" mindset. They have to justify that 60$ pricetag, and people expect the world of their products. this product has to appeal to everyone and their dog, so in the end it has to look almost perfect.
Thus what would have costed 5000$ under a "good enough" mindset and would have looked charmingly retro to the small niche of players interested, and pretty crappy to the average AAA-Games Player, costs now 50+ million $ under a "has to look as good as it can" mindset, looking good to even the average AAA-Games Player.
You can bet while the small Indie Shop maybe spent days on getting the simple sprite sheets for the main protagonist ready, and preparing the animations, with the artist and programmer each maybe spending 1-3 days in their part of the work, the AAA devs spent many person months on the main protagonist alone, with a character designer, 3D modeller, maybe a separate 2D Texture artist, a rigger, animator, an actor for moCap, another for voice overs, programmers and testers involved to make sure the result meets the high quality standarts of the AAA business (and the f*ckups of the last few years *cough*Batman games*cough*Assassins Creed Unity*cough* only strengthen that you cannot get quality for a lower price tag).
That can easely run into 100x or more of the work involved for a single asset. Calculate that up to the full project, add the management costs for large teams, double the budget at least to make up for the marketing (which often is more than double the development budget to make sure the game sells enough copies to recoup the cost), and you add at the insane costs of modern AAA games....
